Use the tryLookupXXX methods rather than lookupXXX methods wherever possible

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Introduce File/Root/Child environments from QVT Declarative

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Use a Child rather than dynamic environment for LetExp and IteratorExp parsing

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Change IntegerLiteralExpCS.integerSymbol from Integer to Number to support BigInteger.
Change UnlimitedNaturalLiteralExpCS.integerSymbol from Integer to Number to support BigInteger.
Change RealLiteralExpCS.realSymbol from Double to Number to support BigDecimal.